refactor: fix credo warnings and format code

- Replace Enum.map/2 |> Enum.join/2 with Enum.map_join/3 for efficiency
- Refactor get_existing_form_values to reduce cyclomatic complexity
- Replace length/1 with Enum.empty?/1 for better performance
- Update gettext translations

test/mv_web/member_live/membership_fee_integration_test.exs

@@ -75,7 +75,7 @@ defmodule MvWeb.MemberLive.MembershipFeeIntegrationTest do
         |> Ash.Query.filter(member_id == ^member.id)
         |> Ash.read!()
 
-      if length(cycles) > 0 do
+      if !Enum.empty?(cycles) do
         cycle = List.first(cycles)
 
         # Change status